1. Field of the Invention
The present invention relates to an optical module. In particular, it relates an optical module having an optical coupling between an optical fiber and a laser diode.
2. Description of the Related Art
In a conventional optical module using a passive alignment mounting technique, it is important to ensure accuracy of the relative positions of a laser diode (LD) chip and an optical fiber at a sub-micron level.
Conventionally, therefore, a V-shape groove 17 (it may be simply called a V groove in this optical technology fields) is formed on a silicon (Si) substrate using anisotropic etching, and a mount mark (not shown) for mounting an LD chip 16 and a slit part 18 are also formed on the Si substrate 15 as shown in FIG. 1. In FIG. 1, an optical fiber 20 is inserted into a ferrule 19.
Referring to FIGS. 2 and 3, the LD chip 16 is mounted in alignment with the mount mark, and a portion of the optical fiber 20 is so mounted onto the V-shape groove 17 as to provide optical coupling with the LD chip 16.
However, in the conventional optical module, while the mounting accuracy of the LD chip 16 on the Si substrate 15 is satisfactory, there is a problem in the mounting of the optical fiber 20 onto the V-shape groove 17 due to a high degree of flexibility of the optical fiber 20.
More specifically, as shown in FIG. 3, since the end of the optical fiber 20 is likely to warp at the slit part 18, it is rather difficult to mount the optical fiber 20 onto the V-shape groove 17 in a consistent state.
Moreover, in common practice of securing the optical fiber 20 to the V-shape groove 17, the optical fiber 20 is temporarily set to the V-shape groove 17 first, and then a downward pressure is applied to the optical fiber 20 on the bevel of the V-shape groove 17 for positioning the optical fiber 20.
At this process, if the V-shape groove 17 has any slight distortion, it is not allowed to accurately secure the optical fiber 20 at a predetermined position, i.e., accuracy of relative positions of the LD chip 16 and the optical fiber 20 cannot be attained at a sub-micron level, making it impossible to provide desired efficiency of optical coupling.
Even if the V-shape groove 17 is accurately formed, an end position of the optical fiber 20 may slightly vary depending on a pressing position to be taken or a degree of force to be exerted on the optical fiber 20 which has been temporarily set on the V-shape groove 17, making it difficult to consistently secure the end of the optical fiber 20 at a predetermined accurate position.
Further, in positioning the optical fiber 20 for optical coupling between the LD chip 16 and the optical fiber 20, the end position of the optical fiber 20 in relative relationship with the LD chip 16 is of critical importance. In the conventional technique of directly mounting the optical fiber 20 on the V-shape groove 17, it is difficult to accurately determine the end position of the optical fiber 20, giving rise to a significant problem in terms of workability and yield of optical coupling.
In Japanese Unexamined Patent Publication No. Hei-10-300987, there is disclosed a structure in which a ferrule 23 fixedly attached around an optical fiber 25 is used for optical coupling between an LD chip 22 mounted on an Si substrate 21 and the optical fiber 25 as shown in FIGS. 4 and 5.
In this conventional optical module, the ferrule 23, which is made of zirconia, is fixedly attached around the optical fiber 25, and then the ferrule 23 is directly mounted on the Si substrate 21 for providing optical coupling with the LD chip 22. There is no groove formed on the Si substrate 21, on the other hand, a notch part 24 is formed at the end of the ferrule 23. A space between the LD chip 22 and the ferrule 23 is filled with a refractive index matching material 26.
According to the conventional optical module, since the zirconia ferrule 23 which is sufficiently lower in flexibility than the optical fiber 25 is mounted on the Si substrate 21, it is possible to enhance workability and stability with respect to a mounting position on the Si substrate 21.
In the conventional optical module, however, since the notch part 24 is formed at the end of the zirconia ferrule 23, there arises a problem of a substantial increase in fabrication cost of the ferrule 23.
More specifically, it is rather difficult to form the notch part 24 because of poor working property of zirconia. For use in practice, the side face of the ferrule 23 must be finished with high precision. Since it is required to perform a process step of forming the notch part 24 before a process step of polishing the side face of the ferrule 23, complexity occurs in fabrication of the ferrule 23. In addition, it becomes difficult to handle the ferrule 23 having the notch part 24 formed thereon.
Further, when mounting the ferrule 23 on the Si substrate 21, it is required to let the notch part 24 of the ferrule 23 meet a flat surface area of the Si substrate 21. Therefore, the ferrule 23 must be adjusted properly in a rotational direction at the time of mounting, whereby the mounting process becomes more complex.
Moreover, though accuracy is required in formation of the notch part 24 at a sub-micron level, it is extremely difficult to ensure such high accuracy in fabrication of the ferrule 23 made of zirconia having poor working property. Still further, in this conventional optical module, a crosswise position reference is not available since no V-shape groove is formed on the Si substrate 21. Therefore, it is impossible to attain crosswise positioning accuracy as high as that in the use of a V-shape groove.
Contrary to intended improvement in workability of mounting on the Si substrate 21, the above problem would cause a decrease in mounting accuracy and an increase in fabrication cost.
Further, another conventional optical module is disclosed in Japanese Unexamined Patent Publication No. Hei 9-105839, wherein it has been proposed to provide a structure in which an optical fiber fixed in a stepped ferrule is set to a V-shape groove formed on an Si substrate and then an LD chip is securely positioned with high accuracy at the end of the stepped ferrule. According to this conventional optical module, it is possible to carry out the assembling and fixing of an optical fiber easily in a simple manner.
However, in this conventional optical module, since the ferrule has a stepped form, a thick-diameter part of the ferrule, which is coupled with an external optical connector, and a thin-diameter part of the ferrule, which is mounted on the V-shape groove, must be formed so that both of them are aligned with respect to center axes of the optical fiber and the ferrule.
Therefore, complexity occurs in fabrication of the ferrule, which would cause a substantial increase in fabrication cost and a decrease in yield.
The stepped structure of the ferrule in this conventional optical module is intended to prevent a stress from being applied to a retention part on the V-shape groove when a spring force is exerted from an external connector. In other words, since a stress is likely to concentrate at a stepped part of the ferrule, there is a possibility that the stepped ferrule may be damaged by a spring force exerted from the external connector.
It is therefore an object of the present invention to provide an optical module and a method of making thereof capable of enhancing mounting accuracy of a ferrule.
Another object of the present invention is to provide an optical module and a method of making thereof capable of enhancing yield in making an optical module.
Still another object of the present invention is to provide an optical module and a method of making thereof capable of reducing fabrication cost.
In order to achieve the above objects, an optical module according to an embodiment of the present invention comprises a substrate on which a groove is formed, a laser diode disposed on the substrate, an optical fiber, and a ferrule with a substantially constant cross section , wherein the optical fiber is inserted into the ferrule and the ferrule is disposed in the groove of the substrate to optically couple the optical fiber with the laser diode.
The groove may be V-shaped and the ferrule a cylindrical shape, the outside diameter of which is substantially constant.
In order to achieve the above objects, a method of making an optical module according to an embodiment of the present invention comprises forming a groove on a substrate, mounting a laser diode on the substrate, mounting a ferrule in the groove of the substrate to optically couple the optical fiber with the laser diode, wherein the ferrule has a cylindrical pipe shape the outside diameter of which is substantially a constant, and an optical fiber is inserted into the ferrule.